A) Field of the Invention
The present invention relates to a solid state image pickup device and an image pickup apparatus having a white balancing function.
B) Description of the Related Art
Most of recent image pickup apparatuses such as video cameras, digital still cameras, and those digital cameras mounted on portable phones and personal computers utilize CCD (charge coupled device) type solid state image pickup devices or MOS (metal-oxide-semiconductor) type solid state image pickup devices as area image sensors.
In both of CCD and MOS type solid state image pickup devices, upon incidence of light, output signals (pixel signals) are generated from electric charges accumulated in a number of photoelectric conversion elements disposed in the surface layer of a semiconductor substrate in a matrix shape in rows and columns. In most of solid state image pickup devices, an output signal (pixel signal) generating unit is fabricated on the same semiconductor substrate on which photoelectric conversion elements are formed.
The output signal generating unit is classified into two types. In one type, charges accumulated in charge transfer units made of CCDs are transferred to a charge detector circuit which generates output signals. CCD is formed by forming a charge transfer channel in the surface layer of a semiconductor substrate and disposing a number of electrodes on an electrically insulating film.
In the other type, electric charges accumulated in a photoelectric conversion element are transferred to an output signal line via a MOS transistor circuit to generate a current or voltage output signal.
A single plate solid state image pickup device for taking a color image has generally a color filter array disposed above photoelectric conversion elements. The color filter array includes color filters of three or more colors. One color filter is provided for each photoelectric conversion element. One photoelectric conversion element and one color filer disposed above the conversion element constitute one pixel.
In a color image pickup apparatus, pixel signals output from the solid state image pickup device are subjected to color separation to generate red, green and blue signals representative of red, green and blue color information of an object to be photographed. These signals are subjected to gamma correction to generate a luminance signal Y and red and blue color difference signals Cr and Cb.
The spectral distribution of light reflected from the surface of an object or the spectral distribution of light transmitted through an object changes greatly with the spectrum distribution of light applied to the object. A person recognizes the object from the reflected or transmitted light.
The color of an object sensed by a person is not determined only by the spectrum distribution of reflected or transmitted light. Each person unconsciously corrects the color based upon ambient light. For this reason, even if the spectral distribution of light applied to an object changes greatly, the color of the object sensed by the person does not change so much.
A color image pickup apparatus is generally equipped with an auto white balance function in order to form an image having a color as near as possible to a color sensed by the person. The auto white balance function reproduces the original color, white as white and gray as gray, independently from the type of illuminating light (including sun light) applied to an object. The levels of blue, green and red signals generated through color separation are controlled in accordance with the type of illuminating light to make red, green and blue signals have the same values when white or gray is photographed.
It is important to distinguish between the types of illuminating light of an object in order to properly operate the auto white balance function.
For example, in an image pickup apparatus having a solid image pickup device with primary color filters, a ratio R/G between red and green signals and a ratio B/G between blue and green signals are calculated and the type of illuminating light is distinguished in accordance with the two-dimensional distribution of R/G and B/G ratios. The R/G and B/G ratios are calculated for the whole frame of a photographed scene or for each of divided areas of the whole frame.
This distinguishment assumes that even if the scene contains a variety of objects, the average of these colors is gray.
For example, even if the white balance is adjusted correctly, the color of a reproduced image of a person illuminated with a standard white fluorescent lamp (a preheating type straight tube white fluorescent lamp for general illumination) becomes distorted. In order to reduce such distortion, it is necessary to perform adjustment other than the adjustment of white balance.
It is therefore desired to precisely judge whether illumination light applied to an object is from a standard white fluorescent lamp.
With conventional distinguishment, however, it is often difficult to correctly distinguish the illumination light of a scene between sunlight and fluorescent light. For example, it is difficult to distinguish between leaf green under dim sunlight and gray under light of a general white fluorescent lamp or three-wavelength type fluorescent lamp. Therefore it is also difficult to judge a light source of a scene of these.